I just bought a 1975 165 hp straight 6 merccruiser in a 21‘ Manatee boat. It‘s a Chevy 6 cylinder that Merccruiser modified for boating purposes. It runs perfect but since it‘s so old do I need to use a lead additive in the gas? I can‘t ask the previous owner as it was purchased at an estate sale.
Nope. Your Mercruiser IL 6 was designed to run on 87 octane unleaded fuel. The elimination of leaded gasoline began in 1974, GMC and Mercruiser where already producing unleaded tolerant engines in 1972. Use no additives, unless you use your boat less than once a week. If you use your boat infrequently, it's a good idea to add the appropriate amount of fuel stabilizer, such as Mercurys' Stor-Clean. If you are using ethanol enhanced gasoline (E10, E15) , it's a good idea to install a water separating filter between the tank and the fuel pump. Because ethanol can erode older types of rubber that where used in fuel lines and fuel pump diaphragms, you might also add a clear filter between the fuel pump and the carburetor. If you have any rubber erosion, you will see black stuff in the filter, and you will need to upgrade your fuel lines and fuel pump.
Harry Buttcrack's answer is excellent. The only thing that I would add is that since you bought it at an estate sale, you can still talk to the original owner, even though he's dead. You just have to yell really, really loud, thats all!
